Message-ID: <CAA8EJpphegHmBFgH1-n9PEkrr-Ys+HCvekKGNYRp=xQxgmC0Cw@mail.gmail.com>
Date: Mon, 16 Sep 2024 07:29:35 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Sandor Yu <sandor.yu@....com>
Cc: Maxime Ripard <mripard@...nel.org>, "andrzej.hajda@...el.com" <andrzej.hajda@...el.com>, 
	"neil.armstrong@...aro.org" <neil.armstrong@...aro.org>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, "jonas@...boo.se" <jonas@...boo.se>, 
	"jernej.skrabec@...il.com" <jernej.skrabec@...il.com>, "airlied@...il.com" <airlied@...il.com>, 
	"daniel@...ll.ch" <daniel@...ll.ch>, "robh+dt@...nel.org" <robh+dt@...nel.org>, 
	"krzysztof.kozlowski+dt@...aro.org" <krzysztof.kozlowski+dt@...aro.org>, 
	"shawnguo@...nel.org" <shawnguo@...nel.org>, "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>, 
	"festevam@...il.com" <festevam@...il.com>, "vkoul@...nel.org" <vkoul@...nel.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-phy@...ts.infradead.org" <linux-phy@...ts.infradead.org>, 
	"kernel@...gutronix.de" <kernel@...gutronix.de>, dl-linux-imx <linux-imx@....com>, 
	Oliver Brown <oliver.brown@....com>, 
	"alexander.stein@...tq-group.com" <alexander.stein@...tq-group.com>, "sam@...nborg.org" <sam@...nborg.org>
Subject: Re: [EXT] Re: [PATCH v16 4/8] drm: bridge: Cadence: Add MHDP8501
 DP/HDMI driver

On Fri, 13 Sept 2024 at 11:46, Sandor Yu <sandor.yu@....com> wrote:
>
>
> > Subject: Re: [EXT] Re: [PATCH v16 4/8] drm: bridge: Cadence: Add MHDP8501
> > DP/HDMI driver
> >
> > On Fri, Sep 06, 2024 at 02:50:08AM GMT, Sandor Yu wrote:
> > > > On Tue, Sep 03, 2024 at 06:07:25AM GMT, Sandor Yu wrote:
> > > > > > -----Original Message-----
> > > > > > From: dri-devel <dri-devel-bounces@...ts.freedesktop.org> On
> > > > > > Behalf Of Maxime Ripard
> > > > > > Sent: 2024年7月2日 21:25
> > > > > > To: Sandor Yu <sandor.yu@....com>
> > > > > > Cc: dmitry.baryshkov@...aro.org; andrzej.hajda@...el.com;
> > > > > > neil.armstrong@...aro.org; Laurent Pinchart
> > > > > > <laurent.pinchart@...asonboard.com>; jonas@...boo.se;
> > > > > > jernej.skrabec@...il.com; airlied@...il.com; daniel@...ll.ch;
> > > > > > robh+dt@...nel.org; krzysztof.kozlowski+dt@...aro.org;
> > > > > > shawnguo@...nel.org; s.hauer@...gutronix.de;
> > festevam@...il.com;
> > > > > > vkoul@...nel.org; dri-devel@...ts.freedesktop.org;
> > > > > > devicetree@...r.kernel.org;
> > > > > > linux-arm-kernel@...ts.infradead.org;
> > > > > > linux-kernel@...r.kernel.org; linux-phy@...ts.infradead.org;
> > > > > > kernel@...gutronix.de; dl-linux-imx <linux-imx@....com>; Oliver
> > > > > > Brown <oliver.brown@....com>; alexander.stein@...tq-group.com;
> > > > > > sam@...nborg.org
> > > > > > Subject: [EXT] Re: [PATCH v16 4/8] drm: bridge: Cadence: Add
> > > > > > MHDP8501 DP/HDMI driver
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > There's still the scrambler issue we discussed on v15, but I
> > > > > > have some more comments.
> > > > > >
> > > > > > On Tue, Jul 02, 2024 at 08:22:36PM GMT, Sandor Yu wrote:
> > > > > > > +enum drm_connector_status cdns_mhdp8501_detect(struct
> > > > > > > +cdns_mhdp8501_device *mhdp) {
> > > > > > > +   u8 hpd = 0xf;
> > > > > > > +
> > > > > > > +   hpd = cdns_mhdp8501_read_hpd(mhdp);
> > > > > > > +   if (hpd == 1)
> > > > > > > +           return connector_status_connected;
> > > > > > > +   else if (hpd == 0)
> > > > > > > +           return connector_status_disconnected;
> > > > > > > +
> > > > > > > +   dev_warn(mhdp->dev, "Unknown cable status, hdp=%u\n",
> > hpd);
> > > > > > > +   return connector_status_unknown; }
> > > > > > > +
> > > > > > > +static void hotplug_work_func(struct work_struct *work) {
> > > > > > > +   struct cdns_mhdp8501_device *mhdp = container_of(work,
> > > > > > > +                                                struct cdns_mhdp8501_device,
> > > > > > > +                                                hotplug_work.work);
> > > > > > > +   enum drm_connector_status status =
> > > > cdns_mhdp8501_detect(mhdp);
> > > > > > > +
> > > > > > > +   drm_bridge_hpd_notify(&mhdp->bridge, status);
> > > > > > > +
> > > > > > > +   if (status == connector_status_connected) {
> > > > > > > +           /* Cable connected  */
> > > > > > > +           DRM_INFO("HDMI/DP Cable Plug In\n");
> > > > > > > +           enable_irq(mhdp->irq[IRQ_OUT]);
> > > > > > > +   } else if (status == connector_status_disconnected) {
> > > > > > > +           /* Cable Disconnected  */
> > > > > > > +           DRM_INFO("HDMI/DP Cable Plug Out\n");
> > > > > > > +           enable_irq(mhdp->irq[IRQ_IN]);
> > > > > > > +   }
> > > > > > > +}
> > > > > >
> > > > > > You shouldn't play with the interrupt being enabled here:
> > > > > > hotplug interrupts should always enabled.
> > > > > >
> > > > > > If you can't for some reason, the reason should be documented in
> > > > > > your
> > > > driver.
> > > > >
> > > > > iMX8MQ have two HPD interrupters, one for plugout and the other
> > > > > for plugin, because they could not be masked, so we have to enable
> > > > > one and
> > > > disable the other.
> > > > > I will add more comments here.
> > > >
> > > > Right, but why do you need to enable and disable them? Do you get
> > > > spurious interrupts?
> > >
> > > They don't have status registers and cannot be masked. If they are not
> > > disabled, they will continuously generate interrupts. Therefore, I have to
> > disable one and enable the other.
> >
> > Sorry, I still don't get it. How can it be useful to detect hotplug interrupts if it
> > constantly sends spurious interrupts when it's enabled?
>
> Yes, this interrupt is different from a normal one; it's likely a design flaw.
> For instance, the plugin interrupt is continuously generated as long as the cable is plugged in,
> only stopping when the cable is unplugged.
> That's why two interrupts are used to detect cable plugout and plugin separately.
> If interrupts aren't used, the only option is polling.

I think I've seen such strange design on other platforms, level
interrupt for HPD, which needs to be disabled via disable_irq().

>
> >
> > > > > > > +   /* Mailbox protect for HDMI PHY access */
> > > > > > > +   mutex_lock(&mhdp->mbox_mutex);
> > > > > > > +   ret = phy_init(mhdp->phy);
> > > > > > > +   mutex_unlock(&mhdp->mbox_mutex);
> > > > > > > +   if (ret) {
> > > > > > > +           dev_err(dev, "Failed to initialize PHY: %d\n", ret);
> > > > > > > +           goto clk_disable;
> > > > > > > +   }
> > > > > > > +
> > > > > > > +   /* Mailbox protect for HDMI PHY access */
> > > > > > > +   mutex_lock(&mhdp->mbox_mutex);
> > > > > > > +   ret = phy_set_mode(mhdp->phy, phy_mode);
> > > > > > > +   mutex_unlock(&mhdp->mbox_mutex);
> > > > > > > +   if (ret) {
> > > > > > > +           dev_err(dev, "Failed to configure PHY: %d\n", ret);
> > > > > > > +           goto clk_disable;
> > > > > > > +   }
> > > > > >
> > > > > > Why do you need a shared mutex between the phy and HDMI
> > controller?
> > > > >
> > > > > Both PHY and HDMI controller could access to the HDMI firmware by
> > > > > mailbox, So add mutex to avoid race condition.
> > > >
> > > > That should be handled at either the phy or mailbox level, not in
> > > > your hdmi driver.
> > >
> > > In both HDMI driver and PHY driver, every mailbox access had protected
> > > by its owns mutex. However, this mutex can only protect each mailbox
> > > access within their respective drivers, and it cannot provide
> > > protection for access between the HDMI and PHY drivers.
> > >
> > > The PHY driver only provides two API functions, and these functions
> > > are only called in the HDMI driver. Therefore, when accessing these
> > > functions, we use a mutex to protect them. This ensures that mailbox
> > > access is protected across different PHY and HDMI drivers.
> >
> > It's really about abstraction. You're using a publicly defined API, and change
> > the semantics for your driver only, and that's not ok.
> >
> > Why can't the mailbox driver itself serialize the accesses from any user, HDMI
> > and PHY drivers included?
> >
>
> In the current code implementation, cdns-mhdp-helper.c isn't a standalone driver but rather a library.
> It provides fundamental mailbox access functions and basic register read/write operations that rely on the mailbox.
> These functions are highly reusable across MHDP8501 and MHDP8546 and can be leveraged by future MHDP versions.
>
> However, most MHDP firmware interactions involve a sequence of mailbox accesses, including sending commands and receiving firmware responses.
> These commands constitute a significant portion of all firmware interactions, encompassing operations like EDID reading and DP link training.
> Unfortunately, these commands cannot be reused between MHDP8501 and MHDP8546.
>
> Creating a dedicated mailbox driver with its own mutex would effectively address race conditions.
> However, this would necessitate relocating all mailbox-related functions to this driver.
> Including these non-reusable functions would defeat the purpose of code reuse.
>
> To strike a balance between code reusability and race condition mitigation, adding mutexes to PHY access functions seems like a reasonable solution.

You seem to have two kinds of scenarios when talking to MHDP: just
cdns_mhdp_mailbox_send(), no response needed and then the
cdns_mhdp_mailbox_send() /  cdns_mhdp_mailbox_recv_header() /
cdns_mhdp_mailbox_recv_data() sequence. Extract those + the mutex
access to separate functions, add a mutex to those sequences and use
them as a high-level API for your HDMI and PHY drivers.

Adding mutexes around phy_foo() calls doesn't look like a proper
solution _at_ _all_.

>
> Sandor
>
> > > > > >
> > > > > > > +static enum drm_mode_status
> > > > > > > +cdns_hdmi_tmds_char_rate_valid(const struct drm_bridge *bridge,
> > > > > > > +                          const struct drm_display_mode *mode,
> > > > > > > +                          unsigned long long tmds_rate) {
> > > > > > > +   struct cdns_mhdp8501_device *mhdp =
> > bridge->driver_private;
> > > > > > > +   union phy_configure_opts phy_cfg;
> > > > > > > +   int ret;
> > > > > > > +
> > > > > > > +   phy_cfg.hdmi.tmds_char_rate = tmds_rate;
> > > > > > > +
> > > > > > > +   /* Mailbox protect for HDMI PHY access */
> > > > > > > +   mutex_lock(&mhdp->mbox_mutex);
> > > > > > > +   ret = phy_validate(mhdp->phy, PHY_MODE_HDMI, 0,
> > &phy_cfg);
> > > > > > > +   mutex_unlock(&mhdp->mbox_mutex);
> > > > > > > +   if (ret < 0)
> > > > > > > +           return MODE_CLOCK_RANGE;
> > > > > > > +
> > > > > > > +   return MODE_OK;
> > > > > > > +}
> > > > > > > +
> > > > > > > +static enum drm_mode_status
> > > > > > > +cdns_hdmi_bridge_mode_valid(struct drm_bridge *bridge,
> > > > > > > +                       const struct drm_display_info *info,
> > > > > > > +                       const struct drm_display_mode *mode) {
> > > > > > > +   unsigned long long tmds_rate;
> > > > > > > +
> > > > > > > +   /* We don't support double-clocked and Interlaced modes */
> > > > > > > +   if (mode->flags & DRM_MODE_FLAG_DBLCLK ||
> > > > > > > +       mode->flags & DRM_MODE_FLAG_INTERLACE)
> > > > > > > +           return MODE_BAD;
> > > > > > > +
> > > > > > > +   /* MAX support pixel clock rate 594MHz */
> > > > > > > +   if (mode->clock > 594000)
> > > > > > > +           return MODE_CLOCK_HIGH;
> > > > > >
> > > > > > This needs to be in the tmds_char_rate_valid function
> > > > > This clock rate check is covered by function
> > > > > tmds_char_rate_valid() It could be removed if keep function
> > > > > tmds_char_rate_valid() be called by
> > > > mode_valid.
> > > >
> > > > Yeah, it's not something you should have to duplicate.
> > > >
> > > > > >
> > > > > > > +   if (mode->hdisplay > 3840)
> > > > > > > +           return MODE_BAD_HVALUE;
> > > > > > > +
> > > > > > > +   if (mode->vdisplay > 2160)
> > > > > > > +           return MODE_BAD_VVALUE;
> > > > > > > +
> > > > > > > +   tmds_rate = mode->clock * 1000ULL;
> > > > > > > +   return cdns_hdmi_tmds_char_rate_valid(bridge, mode,
> > > > > > > +tmds_rate);
> > > > > >
> > > > > > It will already be called by the core so this is redundant.
> > > > >
> > > > > mode_valid function is use to filter the mode list in
> > > > > drm_helper_probe_single_connector_modes(),
> > > > > if function cdns_hdmi_tmds_char_rate_valid() is not called,
> > > > > unsupported
> > > > modes will in mode list.
> > > >
> > > > It's probably something we should deal with in the core somehow. I'm
> > > > not entirely sure how to reconcile drm_bridge_connector and the hdmi
> > > > framework there, but we should at the very least provide a
> > > > mode_valid helper for bridges.
> > >
> > > I agree with that. In fact, I'm a bit confused about the current
> > > mode_valid and tmds_char_rate_valid functions. Ideally, we should find
> > > a way to make tmds_char_rate_valid also work for filtering out the
> > > mode list, rather than just during atomic_check.
> >
> > Yeah, definitely. The way we did so on vc4 for example was to compute the
> > rate for a 8bpc, RGB, output and try to validate that. I think it would be
> > reasonable to start with that.

+1, please extract this code as a helper. You can even submit it
separately, reworking sun4i and vc4 to use a new helper.

-- 
With best wishes
Dmitry

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